In a tangentially related question, Perdix allows the setting of OC Min PPO2 which, I believe, is for trimix users. I have set mine to 0.21 since I will be using air or nitrox and PPO2 should never be below 0.21. Would a trimix diver set this to 0.16 which is (commonly?) held to be the minimum safe PPO2 or would a lower value be chosen? Any insights?
Perdix AI - Different PPO2 for different gases
- Thread starter CWK
- Start date
Please register or login
Welcome to ScubaBoard, the world's largest scuba diving community. Registration is not required to read the forums, but we encourage you to join. Joining has its benefits and enables you to participate in the discussions.
Benefits of registering include
- Ability to post and comment on topics and discussions.
- A Free photo gallery to share your dive photos with the world.
- You can make this box go away
I don't mean to sound snarky, and I'm glad that the OP asked the question.
But the important thing in this discussion is the difference in mindset between technical dive planning and recreational planning. At this level of diving, I don't think that a diver should be using a computer to either alert them to having exceeded the MOD or when to switch gasses (i.e when there is a better deco gas present).
Technical diving is all about planning ahead of time and about situational awareness. Allowing the computer to work like this implies some sort of reliance on it, which (especially during training) isn't a great idea. You plan your dive with your bottom gas and your deco gas, and you know ahead of time what your MOD is and your switch depth(s) are. And you don't rely on your computer to tell you either of those things
Also, I agree with the others - a cylinder marked "OXYGEN" has 100% O2 in it . All deco mixes have the MOD in big letters, and usually the EAN percentage in smaller print.
But the important thing in this discussion is the difference in mindset between technical dive planning and recreational planning. At this level of diving, I don't think that a diver should be using a computer to either alert them to having exceeded the MOD or when to switch gasses (i.e when there is a better deco gas present).
Technical diving is all about planning ahead of time and about situational awareness. Allowing the computer to work like this implies some sort of reliance on it, which (especially during training) isn't a great idea. You plan your dive with your bottom gas and your deco gas, and you know ahead of time what your MOD is and your switch depth(s) are. And you don't rely on your computer to tell you either of those things
Also, I agree with the others - a cylinder marked "OXYGEN" has 100% O2 in it . All deco mixes have the MOD in big letters, and usually the EAN percentage in smaller print.
@doctormike,
I understand where you are coming from and I agree with your view.
I would like to know what my computer implements the theory that is set out in the manuals and this little discussion has helped me understand Perdix a little better. This is not to say that I depend on the computer to generate alarms for having exceeded MOD or to switch gases but I am now certainly aware of where the settings for these alarms are.
I understand where you are coming from and I agree with your view.
I would like to know what my computer implements the theory that is set out in the manuals and this little discussion has helped me understand Perdix a little better. This is not to say that I depend on the computer to generate alarms for having exceeded MOD or to switch gases but I am now certainly aware of where the settings for these alarms are.
Two things:
(1) writing OXYGEN on anything other than a 6m/20ft tank is a BAD idea. As others have said.
(2) In answer to the OP in post #11, you may be confused by the difference between the fraction of O2 in the gas, and the Partial Pressure of the O2 in the gas. A deep trimix diver might have (for example) just 10% O2 in his gas, but at (say) 10m/33ft that is a partial pressure of 0.20 ATM, perfectly breathable. You just can't breath that gas near/at the surface.
(1) writing OXYGEN on anything other than a 6m/20ft tank is a BAD idea. As others have said.
(2) In answer to the OP in post #11, you may be confused by the difference between the fraction of O2 in the gas, and the Partial Pressure of the O2 in the gas. A deep trimix diver might have (for example) just 10% O2 in his gas, but at (say) 10m/33ft that is a partial pressure of 0.20 ATM, perfectly breathable. You just can't breath that gas near/at the surface.
I don't think that the OP has confused PO2 and FiO2. The question in post #11 (as I read it) was whether a diver with a hypoxic mix would set an alarm to tell them if they had ascended above the point where the mix was safe to breathe. Alarms are to track values that vary (unlike FiO2 on a given OC mix).
As I mentioned, I don't think that a diver at that level should be using a computer to monitor this, but I see what they are getting at...
@tursiops,
I believe that I understand the theory behind using hypoxic gas in diving. The very fact that Perdix has a Min PO2 setting suggests to me that they are allowing a setting below 0.21. I would imagine that the Min PO2 would be set at 0.16 or 0.17.
To me, the Min PO2 in conjunction with the Max PO2 would set the operational range for the hypoxic gas. Nomoxic and hyperoxic gas would not require the Min PO2 setting.
At a Min PO2 setting of 0.16, Perdix should allow a hypoxic gas with 8% O2 to be used at 10M.
@doctormike,
Perdix will monitor Min PO2 and Max PO2 irrespective of whether you and I change the settings. If you don't change the settings, Perdix will just use default values. Those alarms are set. I don't believe that Perdix allows you to turn the alarms off. Understanding the theory, knowing where to find the settings, and having sufficient knowledge to change the settings away from default values does not mean that you are in any way more dependant on the computer. It just means that you know what the computer is doing and can change the default values to set it to be more or less conservative.
As someone certified 30 years ago before computers were readily available, I would be quite comfortable diving with a depth guage, bottom timer, and RDP. In fact, I used to have a credit card sized RDP that followed me diving. I believe that I would be able to execute a tec dive with a depth guage, bottom timer and dive plan (together with contingency dive plans in case planned depth and/or time are exceeded). Having said this, I find computers to be empowering.
I believe that I understand the theory behind using hypoxic gas in diving. The very fact that Perdix has a Min PO2 setting suggests to me that they are allowing a setting below 0.21. I would imagine that the Min PO2 would be set at 0.16 or 0.17.
To me, the Min PO2 in conjunction with the Max PO2 would set the operational range for the hypoxic gas. Nomoxic and hyperoxic gas would not require the Min PO2 setting.
At a Min PO2 setting of 0.16, Perdix should allow a hypoxic gas with 8% O2 to be used at 10M.
@doctormike,
Perdix will monitor Min PO2 and Max PO2 irrespective of whether you and I change the settings. If you don't change the settings, Perdix will just use default values. Those alarms are set. I don't believe that Perdix allows you to turn the alarms off. Understanding the theory, knowing where to find the settings, and having sufficient knowledge to change the settings away from default values does not mean that you are in any way more dependant on the computer. It just means that you know what the computer is doing and can change the default values to set it to be more or less conservative.
As someone certified 30 years ago before computers were readily available, I would be quite comfortable diving with a depth guage, bottom timer, and RDP. In fact, I used to have a credit card sized RDP that followed me diving. I believe that I would be able to execute a tec dive with a depth guage, bottom timer and dive plan (together with contingency dive plans in case planned depth and/or time are exceeded). Having said this, I find computers to be empowering.
I’m assuming that you are doing your tech training in open circuit. It’s sort of a semantic discussion, but the whole point of a PO2 alarm for an open circuit diver is to have the computer let you know when you have descended below (for any mix) or ascended above (for a hypoxic mix) the maximum or minimum safe operating depth for the mix in your tank. These depths are fixed for a given back gas or deco gas, so the only purpose of having the computer monitor these and alert you is to remind you not to exceed these depth limits. No matter when you were certified, this is essentially what a PO2 alarm is doing for an OC diver - watching your depth.
On CCR, you absolutely need some sort of monitoring (and alarms are helpful) since your PO2 can spike or drop to unsafe levels without changing depth due to a number of reasons beyond the scope of this discussion. So that’s more of the reason for those alarms.
I’m not saying that you shouldn’t know how to set those alarms. I’m saying that for OC diving, you don’t really need them because you know your MOD ahead of time, and you shouldn’t need a computer alarm to keep you above your MOD as a technical diver. You will need to have pretty precise situational awareness and control of your depth to hold stops, etc…
Even for switching to your deco gas, there is a whole procedure for a safe NOTOX switch that involves you knowing what is in the tank and what your depth is, and this doesn’t change based on the actual dive profile. If you want to be more conservative in terms of O2 exposure when you switch to a deco mix, then you should calculate a shallower MOD for that mix ahead of time (1.4 instead of 1.6, for example), label your tank with that, and make your switch at a shallower depth (trading O2 safety for deco efficiency).
Computers are empowering for one reason - they give you decompression “credit” for time spent above your planned square profile bottom depth, because they track a continuously varying value (depth) better than a human can. And in the case of CCR diving, they also track a continuously varying FiO2 with a relatively fixed PO2.
@doctormike,
All I'll say is that the alarms are programmed in by Shearwater. I think that at this level, divers should be aware of how these alarms are set. The fact that the alarms are built into the computer in no way implies that divers are dependant on them.
I have restored factory defaults and the factory default settings are:
Min PPO2 0.19
Max PPO2 1.65
Deco PPO2 1 .61
I'd be interested to hear from Shearwater as to why Max PPO2 defaults to 1.65 instead of 1.40 or 1.41.
All I'll say is that the alarms are programmed in by Shearwater. I think that at this level, divers should be aware of how these alarms are set. The fact that the alarms are built into the computer in no way implies that divers are dependant on them.
I have restored factory defaults and the factory default settings are:
Min PPO2 0.19
Max PPO2 1.65
Deco PPO2 1 .61
I'd be interested to hear from Shearwater as to why Max PPO2 defaults to 1.65 instead of 1.40 or 1.41.
Well, learn something new every day ...
I never realized there was PPO2 settings in the Petrel/Perdix (under Adv Config 2).
I've never worried about it (I do have the centre middle display as PP02). But diving the gases I do, I don't worry about where buy PP02 is sitting on OC. A great thing about standard gases!
I do occasionally take a quick peek at that information at the surface (at elevation) to see how close I am to the hypoxic range with 18/45.
_R
I never realized there was PPO2 settings in the Petrel/Perdix (under Adv Config 2).
I've never worried about it (I do have the centre middle display as PP02). But diving the gases I do, I don't worry about where buy PP02 is sitting on OC. A great thing about standard gases!
I do occasionally take a quick peek at that information at the surface (at elevation) to see how close I am to the hypoxic range with 18/45.
_R
Similar threads
